Implement KHR_node_visibility in the GLTF module

[aaronfranke]

Implements KHR_node_visibility KhronosGroup/glTF#2410 in Godot, which allows glTF nodes to be marked as not visible. Both import and export are supported.

Details: The glTF visibility system works the same as Godot's visibility system, only affecting visuals, which makes this really easy to implement. Since this extension was extremely simple, I decided to just implement it in GLTFDocument rather than writing a GLTFDocumentExtension class for it. I also made it a required extension on export, to ensure that user assets are always rendered correctly (EDIT: Required by default, but users can customize this on export). However most scenes don't contain invisible objects, so this won't impact most scenes.

Aside from the GLTF module changes, this PR also adds 2 lines to the general scene importer code to ensure visibility is preserved when converting ImporterMeshInstance3D nodes to MeshInstance3D nodes.

Test file: cube_visibility_example.zip

[fire]

I mentioned this proposal at the gltf meeting so I agree with it. Did not test yet.

[fire]

We had a lengthy discussion of the support of the visibility extension if we make it required.

Required extension for something cosmetic is a bad idea IMHO

Required extensions will make godot look bad and create a support headache

[aaronfranke]

Updated the PR to make this configurable on export:

[akien-mga]

Shouldn't we wait for Khronos to approve the draft proposal before merging its implementation? If they reject it, but we merge it, we'd be stuck maintaining a non-standard extension.

[aaronfranke]

@akien-mga Indeed, we can wait for approval. I doubt this will change but I cannot absolutely guarantee that.

[fire]

We discussed on Thursday that the moment Khronos ratifies the extension we are clear to merge.

[aaronfranke]

Marking as draft to prevent accidental merging before ratification. However, the code is complete and ready for review (well, it has already been reviewed, but more review is welcome too).

[yankscally]

Honestly, it is only a single bool, its not that deep. and, it has a lot of value to godot when blender and godot share this eye icon in the same panel. I think it's worth supporting and then hotfixing later if Khronos happen to change their mind.